Appendix BProperties of electrolyte solutions

Our interest in flows of electrolyte solutions in microdevices requires that we keep track of electrolyte solutions
themselves, as well as acid-base chemistry at surfaces and in buffers. The coupling of electric fields and fluid
mechanics that is common in microfluidics leads also to coupling between fluid mechanics and chemistry, since
acid-base chemistry describes most interfacial charge and the charge state of many common analytes. Acid-base
reactions at interfaces dictate the interfacial charge and, in turn, the electroosmotic mobility of the interface.
Acid-base reactions, for example, between water and DNA or between water and proteins dictate the electrophoretic
mobility of proteins and DNA in solution.

Because of this, this appendix provides a description of the properties of water, electrolyte solutions, and the
associated acid-base chemistry. We define solution terminology, derive the Henderson-Hasselbach equation for
dissociation equilibrium, show how the Henderson-Hasselbach equation relates reaction pKa to acid dissociation,
and show how the water dissociation equation leads to a simple relation between pH and pOH for water at room
temperature. This basic understanding explains, for example, the pH dependence of the electroosmotic
mobility.